1. Field of the Invention
The present invention relates to the field of data storage and synchronization. In particular it relates to a method for compensating for a class or repeatable disturbances that are very common in the area of rotating machinery, and specifically in the area of rotating storage media on a spindle. In such cases, spindle rotation causes many disturbances to be injected into the loop used to recover signals used for synchronization. A large fraction of these disturbances are harmonic, that is, they occur at a known frequency which is related to the spindle frequency.
2. Art Background
Disk data storage devices feature rotating media with data recorded on tracks on the media. These tracks may be in the form of a plurality of concentric circles, or they may be in the form of a single spiral. Formatting information is present on the media which allows the disk drive to recover the signals needed to read and possibly write information to and from the media. The media is rotated on a spindle. Imperfections in the spindle apparatus, and in the positioning of the media, introduce disturbances into signals read from the media. Many of these disturbances are harmonic in nature, occurring at a known frequency which is related to the spindle frequency.
For fixed magnetic disk drives, in which the tracks are formatted after the media has been secured to the spindle, the dominant harmonics are most closely related to minor defects and tolerances in the spindle itself. However, in removable media, such as an optical storage medium, including but not limited to DVD+RW, the dominant feature is the imprecise positioning of the media and therefore the tracks relative to the true center of the spindle. This positioning offset causes eccentricity in the path which the tracks will take. This means the tracks will not pursue a true circle around the axis of rotation of the spindle; instead, the tracks will have an eccentricity which manifests itself as a set of sinusoidal deviations from the true circular path.
In a storage device using a rotating storage medium, the act of reading or writing data necessitates the generation of a clock signal to keep the data synchronized. Furthermore, this clock must be synchronized to the rotating medium itself, so that the data can be repeatably positioned on, and recovered from, the storage medium. In order to generate a clock for reading or writing, it is common to use a phase-locked loop (PLL) which generates a repeatable clock which uses as its input a reference signal measured from the rotating medium. Phase-locked loops have the general nature that they are feedback loops applied to electronic signals rather than motion control signals.
A multiplicity of loops are commonly used in rotating storage devices. One loop maintains the tracking position of the read/write assembly. Another loop produces the reference clock used for reading and writing data. A third possible loop is used in far field devices, such as rewriteable optical storage, to maintain the height of the readback mechanism or the focus position of the objective lens. Both of the former loops exhibit sensitivity to track eccentricity. The sensitivity of the tracking position loop to the harmonic disturbances described above can be reduced using a variety of methods known to the art. However, even if the distorted track were being followed perfectly, the mere act of following the eccentricity would produce differences in the reference clock period around the circumference of the track.
What is needed is a method for correcting clock recovery loops in the presence of harmonic disturbances.
Disturbances introduced into a phase-locked loop (PLL) by harmonic sources in rotating storage media are reduced by applying harmonic correction. Harmonic correction may be present at all times in the PLL, or may be switched in once loop lock has been obtained. Harmonic correction reduces the resultant noise and jitter of the loop. The nature of the harmonic correction employed depends on the nature of the disturbance, as well as the nature of the loop. In a first embodiment, where the disturbance is well within the PLL bandwidth, an additional integrating pole or a bump (or resonant) filter is added to the loop. In a second embodiment, where the disturbance is well outside the PLL bandwidth, an additional low pass or notch (anti-resonant) filter is added to the loop. In a third embodiment, harmonic correction is obtained by generating a sinusoid or a combination of sinusoids at a phase and frequency so as to cancel out the disturbance; this signal is added as a feedforward signal. In a forth embodiment, harmonic correction is obtained in a repetitive control scheme using a filtered version of the residual errors on previous rotations of the media as a feedforward signal to cancel harmonic effects. These embodiments may be repeated for each harmonic frequency at which a significant disturbance is present, and may be used in combination with each other.